As with latitude, Declination is measured away from the celestial equator.
But there is again no obvious choice for the starting point of the
other set of coordinates.
Where should we start counting Right Ascension?
The rather arbitrary choice made by astronomers long ago was
to pick the point at which the Sun appears to cross the
celestial equator from South to North as it moves through the
sky during the course of a year.
We call that point the "vernal equinox".

Once again, there are several ways to express a location.
The star Sirius, for example, can be described as at

Right Ascension 101.287 degrees, Declination -16.716 degrees

We can also express the Declination in Degrees:ArcMinutes:ArcSeconds,
just as we do for latitude;
and, as usual, there are 360 degrees around a full circle.
For Right Ascension, astronomers always use the convention of
Hours:Minutes:Seconds.
There are 24 hours of RA around a circle in the sky,
because it takes 24 hours for the Sun to move all the
way from sunrise to the next sunrise.

Thus, 1 hour of RA equals 15 degrees of RA.
Along the celestial equator, those 15 "degrees of RA"
are the same as 15 ordinary degrees.
That is, a star located at (RA = 6 hours, Dec = 0 degrees)
would be exactly 15 degrees away from a star located
at (RA = 7 hours, Dec = 0 degrees).
As one moves away from the celestial equator and toward the
poles, the lines of RA converge:

For points off the celestial equator,
the real distance in the RA direction is smaller
by a factor of cos(Dec).